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United States Patent |
6,017,641
|
Aoki
,   et al.
|
January 25, 2000
|
Coil spring resistive to delayed fracture and manufacturing method of
the same
Abstract
A coil spring made of an oil-tempered steel wire with internal hardness of
more than Hv 550 in cross-section, the surface hardness of the
oil-tempered steel wire being determined in an extent between Hv 420 in a
minimum value and hardness defined by subtraction of Hv 50 from the
internal hardness in a maximum value.
Inventors:
|
Aoki; Toshinori (Nagoya, JP);
Nishimura; Taisuke (Wako, JP);
Otowa; Takashi (Wako, JP)
|
Assignee:
|
Chuo Hatsujo Kabshiki Kaisha (Nagoya, JP);
Honda Giken Kogyo Kabushiki Kaisha (Tokyo, JP)
|
Appl. No.:
|
038988 |
Filed:
|
March 12, 1998 |
Current U.S. Class: |
428/544; 148/208; 148/212; 148/215; 148/230; 148/233; 148/580; 148/599; 148/660; 148/901; 148/908; 267/166; 428/592; 428/906 |
Intern'l Class: |
B21F 035/00; C21D 009/02 |
Field of Search: |
428/544,592,906
148/208,210,212,215,230,233,580,596,599,629,660,901,908
267/166
|
References Cited
Foreign Patent Documents |
04285142 | Oct., 1992 | JP.
| |
Other References
Nishimura et al, SAE Technical Paper Series No. 890777, The Engineering
Society for Advancing Mobility Land Sea Air and Space, "The Valve Springs
Carbo-Nitrided at a High Temperature for High Speed Engines",
International Congress and Exposition Detroit, Michigan, Feb. 27-Mar. 3,
1989.
|
Primary Examiner: Jones; Deborah
Assistant Examiner: Koehler; Robert R.
Attorney, Agent or Firm: Nixon & Vanderhye P.C.
Claims
What is claimed is:
1. A coil spring made of an oil-tempered steel wire with internal hardness
of more than Hv 550 in cross-section, the surface hardness of the
oil-tempered steel wire being determined in an extent between Hv 420 in a
minimum value and hardness defined by subtraction of Hv 50 from the
internal hardness in a maximum value.
2. A coil spring as claimed in claim 1, wherein the surface of the
oil-tempered steel wire is decarburized during heating prior to a
quenching process thereof to determine the surface hardness of the
oil-tempered steel wire in the extent between Hv 420 in a minimum value
and harness defined by subtraction of Hv 50 from the internal hardness in
a maximum value.
3. A coil spring as claimed in claim 1, wherein the oil-tempered steel wire
contains 0.45 to 0.8% C. 1.2 to 2.5 % Si, 0.5 to 1.5% Mn and 0.5 to 2.0%
Cr, by weight and at least one metallic element selected from the group
consisting of 0.1 to 0.7% Mo, 0.05 to 0.6% V, 0.2 to 2.0% Ni and 0.01 to
0.2% Nb, by weight and contains Fe and impurity elements as a remainder.
4. A manufacturing method of a coil spring made of an oil-tempered steel
wire with internal hardness of more than Hv 550 in cross-section,
comprising the steps of:
decarburizing the surface of the oil-tempered steel wire during heating
prior to a quenching process thereof to determine the surface hardness of
the oil-tempered steel wire in an extent between Hv 420 in a minimum value
and hardness defined by subtraction of Hv 50 from the internal hardness in
a maximum value; and
coiling the oil-tempered steel wire for making a coil spring.
5. A manufacturing method of a coil spring as claimed in claim 4, further
comprising the steps of:
applying nitriding treatment to the coil spring; and
applying shot peening treatment to the nitrided coil spring.
Description
BACKGROUND-OF THE INVENTION
1. Field of the invention The present invention relates to a high strength
coil spring made of an oil-tempered steel wire, and more Particularly to a
manufacturing method of the coil spring capable of restraining delayed
fracture of the coil spring.
2. Description of the Prior Art
In recent years, it is required to provide lightweight valve springs
adapted for use in automotive engines. To satisfy such requirements, there
have been proposed various methods for strengthening an oil-tempered steel
wire for the valve springs. For example, there has been proposed an
oil-tempered steel wire with tensile strength of more than 210
kgf/mm.sup.2 and internal hardness of more than Hv 550, which contains
0.45 to 0.8% C, 1.2 to 2.5% Si. 0.5 to 1.5% Mn and 0.5 to 2.0% Cr, by
weight and at least one metallic element selected from the group of 0.1 to
0.7% Mo, 0.05 to 0.6% V, 0.2 to 2.0% Ni and 0.01 to 0.2% Nb, by weight and
contains Fe and impurity elements as a remainder.
In the oil-tempered steel wire of this kind, it has been found that there
occur breakage of the steel wire during a cold coiling process and delayed
fracture of the steel wire after the cold coiling process. Disclosed in
Japanese Patent Laid-open Publication No. 4(1992)-285142 is a method of
decarburizing the surface of the steel wire for preventing the steel wire
from breakage during the cold coiling process. The surface hardness of the
steel wire defined by decarburizing treatment prior to the oil-tempering
process is, however, limited to less than Hv 400. For this reason, the
effect of the nitriding treatment for increasing the surface hardness of
the steel wire is reduced, resulting in decrease of fatigue strength of
the valve springs. In addition, for increasing the surface hardness of the
steel wire more than Hv 900 by nitriding treatment in an atmosphere of
ammonia gas, it is required to carry out the nitriding treatment at 500
.degree. C. for more than six hours. This lowers the productivity of the
steel wire. Furthermore, in the oil-tempered steel wire described above,
delayed fracture of the steel wire will occur after the coiling process
due to an increase of retained austenite and an increase of residual
stress on the surface of the steel wire caused by the coiling process.
SUMMARY OF THE INVENTION
It is, therefore, a primary object of the present invention to provide a
high strength coil spring resistive to delayed fracture without causing
any problem discussed above.
According to an aspect of the present invention, the object is accomplished
by providing a coil spring made of an oil-tempered steel wire with
internal hardness of more than Hv 550 in cross-section, the surface
hardness of the oil-tempered steel wire being determined in an extent
between Hv 420 in a minimum value and hardness defined by subtraction of
at least Hv 50 from the internal hardness in a maximum value.
In the coil spring, it is preferable that the oil-tempered steel wire is
decarburized during heating prior to a quenching process thereof in such a
manner that the surface hardness of the oil-tempered steel wire is
determined in the extent between Hv 420 in a minimum value and hardness
defined by subtraction of at least Hv 50 from the internal hardness in a
maximum value.
According to another aspect of the present invention, the object is
accomplished by providing a manufacturing method of a high strength coil
spring made of an oil-tempered steel wire with internal hardness of more
than Hv 550 in cross-section, comprising the steps of decarburizing the
surface of the oil-tempered steel wire during hearing prior to a quenching
process thereof to determine the surface hardness of the oil-tempered
steel wire in an extent between Hv 420 in a minimum value and hardness
defined by subtraction of Hv 5 from the internal hardness in a maximum
value, and coiling the oil-tempered steel wire for making a coil spring.
BRIEF DESCRIPTION OF TEE DRAWINGS
In the drawings:
FIG. 1 is a graph showing retained austenite on the surface of each of a
sample steel wire and comparative steel wires in relation to the surface
hardness of each of the steel wires before a coiling process thereof;
FIG. 2 is a graph showing residual stress on the surface of each the sample
steel wire and comparative steel wires after the coiling process in
relation to the surface hardness of each of the steel wires before the
coiling process thereof;
FIG. 3 is a graph showing residual stress on the surface of each of the
sample steel wire and comparative steel wires after the coiling process in
relation to an occurrence time of delayed fracture;
FIG. 4 is a graph showing the surface hardness of each of the sample steel
wire and comparative steel wires after a nitriding process thereof in
relation to the surface hardness of each of the same steel wires before
the coiling process thereof;
FIG. 5 (a) is a graph showing the hardness of the sample steel wire before
the coiling process thereof and after the nitriding process thereof in
relation to depth from the surface of the sample steel wire;
FIG. 5(b) is a graph showing the hardness of each of the comparative steel
wires before the coiling process thereof and after the nitriding process
thereof in relation to depth from the surface of the comparative steel
wires; and
FIG. 6 is a graph showing test results of durability of the sample steel
wires in comparison with the comparative steel wires.
DESCRIPTION OF TEE PREFERRED EMBODIMENT
Hereinafter, a preferred embodiment of the present invention will be
described in detail on a basis of an experiment. In the following Table 1,
there is illustrated each chemical composition of sample steel wires (1)
to (5) of the present invention and comparative steel wires (I) to (III)
adapted for an experiment in the preferred embodiment. As is understood
from Table 1. each chemical composition of the sample steel wires (1) to
(5) is essentially the same as each chemical composition of the
comparative steel wires.
TABLE 1
______________________________________
C Si Mn Cr Mo V Ni Nb
______________________________________
Sample wire
0.73 2.01 0.75 1.02 0.22 0.37 -- 0.02
(1)
Sample wire
0.75 2.01 0.79 0.79 0.21 0.45 -- 0.02
(2)
Sample wire
0.75 2.00 0.71 1.27 0.21 0.27 -- 0.02
(3)
Sample wire
0.71 1.42 0.61 0.58 0.13 0.43 -- --
(4)
Sample wire
0.75 2.01 0.75 1.02 0.22 0.37 1.0 0.02
(5)
Comparative
0.73 2.01 0.75 1.02 0.22 0.37 -- 0.02
wire (I)
Comparative
0.73 2.01 0.75 1.02 0.22 0.37 -- 0.02
wire (II)
Comparative
0.71 1.42 0.61 0.58 0.13 0.43 -- --
wire (III)
______________________________________
In the following Table 2, there are shown each condition for oil-tempering
the sample steel wire (1) the surface of which was decarburized by a
method of the present invention and for oil-tempering the comparative
steel wires (I) to (III) used without the decarburizing process and
tensile strength of each of the steel wires (1) and (I) to (III) after
treatment of the oil-tempering. Only the steel wire (1) was heated for
quenching in an electric furnace filled with inert gas such as argon gas
and decarburized in an atmosphere of mixed gases of argon, hydrogen and
air. The oxidation and decarburization of the sample steel wire (1) were
adjusted in accordance with change of a dew point, and the dew point was
controlled by the amount of air.
TABLE 2
______________________________________
Condition for oil-tempering
Tensile Quenching Tempering
strength Temp. Temp. Atmosphere
______________________________________
Sample wire
230 kgf/mm.sup.2
930.degree. C.
480.degree. C.
H, Ar, Air
(1)
Comparative
230 kgf/mm.sup.2
930.degree. C.
500.degree. C.
Ar
wire (I)
Comparative
220 kgf/mm.sup.2
930.degree. C.
500.degree. C.
Ar
wire (II)
Comparative
210 kgf/mm.sup.2
930.degree. C.
480.degree. C.
Ar
wire (III)
______________________________________
The sample steel wire (1) and comparative steel wires (I) to (III) each
were formed as a rod of 3.4mm in diameter by cold drawing and applied with
the treatment of quenching and oil-tempering under each condition listed
in Table 2. The oil-tempered steel wires were coiled as in a specification
listed in the following Table 3 and applied with treatment of nitriding
and shot peening to make a sample coil spring and comparative coil
springs.
TABLE 3
______________________________________
Wire diameter 3.4 mm
Average diameter of coils
19.4 mm
Effective number of windings
4.76
Total number of windings
6.76
Height in free condition
44.6 mm
Spring coefficient 3.97 kgf/mm
______________________________________
Illustrated in FIG. 1 is retained austenite on the surface of each of the
sample steel wire (1) and comparative steel wires (I) to (III) in relation
to the surface hardness of each or the steel wires before the coiling
process. As is understood from FIG. 1, the retained austenite on the
surface of the sample steel wire (1) after heat treatment was decreased as
a result of decarburizing treatment prior to the quenching process, and
the surface hardness of the sample steel wire (1) was decreased. The
retained austenite causes martensite transformation during the coiling
process to increase the surface hardness of the steel wire immediately
after the coil process. This results in delayed fracture of the steel
wire. It is, therefore, desired to reduce the retained austenite on the
surface of the steel wire.
In FIG. 2, there is shown residual stress (MPa) on the surface of the
sample steel wire after the coiling process in relation to the surface
hardness (Hv) of the sample steel wire before the coiling process. As
shown in FIG. 2, it has been found that the residual stress on the surface
of the sample steel wire after the coiling process tends to decrease in
accordance with a decrease of the surface hardness. Since the surface
hardness of the sample steel wire was decreased by decarburization, the
residual stress on the surface of the sample steel wire after the coiling
process was decreased.
Illustrated in FIG. 3 is the residual stress (MPa) on the surface of the
sample steel wire in relation to an occurrence time of delayed fracture in
the case that the steel wire was clamped by stress of 98 MPa in solution
of HCl of 1.896 in gravity. As shown in FIG. 3, it has been found that the
residual stress on the surface of the sample steel wire of Hv 460
decreased less than that of the comparative steel wires (I) of Hv 610
after the coiling process. This implies that the occurrence of delayed
fracture in the sample steel wire is remarkably delayed. Based on the
result, it is assumed that if the residual stress on the surface of the
steel wire after the coiling process is about 700 MPa. any delayed
fracture does not occur even when 100 hours have passed.
In FIG. 4, there is shown the surface hardness of each of the sample and
comparative steel wires nitrided at 500 .degree. C. in the atmosphere of
ammonia gas for two hours in relation to the surface hardness of each of
the oil-tempered steel wires. As shown in FIG. 4, it is has been found
that the surface hardness of each of the nitrided steel wires tends to
decrease in accordance with a decrease of the surface hardness of each of
the steel wires for the following reason. As nitrogen as well as carbon is
an element forming solid solution of the interstitial type, the surface
hardness of the nitrided steel wire is determined by a sum of the amount
of carbon contained in the surface of the steel wire and the amount of
nitrogen invaded into the surface of the steel wire. It is, therefore,
required to prolong the treatment time for nitriding of the steel wire in
accordance with a decrease of the amount of carbon on the surface of the
steel wire caused by decarburizing treatment.
As the durability of coil springs is determined by the surface strength of
the steel wires, the nitriding treatment was carried out to increase the
surface hardness of the coil spring more than Hv 900. In the case that the
nitriding treatment is carried out at 500 .degree. C. to increase the
surface hardness of the coil spring more than Hv 900 and finished within
two hours to enhance productivity of the coil springs, it is required to
retain the surface hardness of the steel wire more than Hv 420 prior to
the nitriding treatment.
In FIGS. 5(A) and 5 (B), the hardness of each of the steel wires and the
hardness (Hv) of each of the coil springs nitrided at 500 .degree. C. in
the atmosphere of ammonia gas for two hours are shown in relation to depth
from the surface or each of the coil spring. As shown in FIG. 5(A), the
surface hardness of the coil spring made of the sample steel wire (1) is
decreased by the decarburizing treatment less than the internal hardness
of the coil spring in an extent of more than Hv 50. This implies that a
decrease of residual stress after the coiling process is effective to
restrain delayed fracture of the coil spring. In the case that the sample
steel wire (1) of more than Hv 420 in surface hardness is used for making
the coil spring, the surface hardness of the nitrided coil spring becomes
more than Hv 900 sufficient for durability of the coil spring. For the
reasons described above, it has been found that delayed fracture of the
coil spring is effectively restrained when the surface hardness of the
oil-tempered steel wire was determined in an extent between Hv 420 in a
minimum value and hardness defined by subtraction of Hv 50 from the
internal hardness of the coil springs in a maximum value. In the case that
the maximum value of the surface hardness of the oil-tempered steel wire
is adjusted in an extent of less than Hv 50, delayed fracture of the coil
spring may not be restrained since the control of the surface hardness
becomes difficult due to errors in carbon content during the decarburizing
process for mass-production.
In FIG. 6, there are shown fatigue test results of tile coil springs made
of the sample steel wire (1) and comparative steel wires (I) to (III).
From the test results, it has been found that the fatigue strength of the
coil spring made of the sample steel wire (1) was increased by the
nitriding treatment in a short period of time in spite of decarburizing
treatment to the surface of the sample steel wire.
Although in the experiment described above, the present invention was
adapted to an oil-tempered steel wire containing 0.45 to 0.8% C. 1.2 to
2.5% Si. 0.5 to 1.5% Mn and 0.5 to 2.0% Cr, by weight and at least one
metallic element selected from the group of 0.1 to 0.7% Mo, 0.05 to 0.6%
V, 0.2 to 2.0% Ni and 0.01 to 0.2% Nb, by weight and containing Fe and
impurity elements as a remainder, the present invention can be effectively
adapted to an oil-tempered steel wire of more than Hv 550 in internal
hardness.
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